The present invention relates to processes for synthesizing borohydride compounds, and more particularly to processes of synthesizing borohydride compounds with reduced energy requirements.
Environmentally friendly fuels (e.g., alternative fuels to hydrocarbon based energy sources) are currently of great interest. One such fuel is borohydride, which can be used directly as an anodic fuel in a fuel cell or as a hydrogen storage medium (e.g., hydrogen can be liberated by the reaction of sodium borohydride with water, which produces sodium borate as a byproduct). As with all fuels, acceptance of borohydride in the commercial market is partially dependent on the availability of industrial scale quantities.
Typical industrial processes for the production of sodium borohydride are based on the Schlesinger process (Equation 1) or the Bayer process (Equation 2), which are both described below. Equation 1 illustrates the reaction of alkali metal hydrides with boric oxide, B2O3, or trimethoxyborate, B(OCH3)3, at high temperatures (e.g., ca. 330 to 350xc2x0 C. for B2O3 and 275xc2x0 C. for B(OCH3)3). These reactions, however, provide poor molar economy by requiring four moles of sodium to produce one mole of sodium borohydride.
4NaH+B(OCH3)3xe2x86x923NaOCH3+NaBH4 xe2x80x83xe2x80x83(1)
Na2B4O7+16Na+8H2+7SiO2xe2x86x924NaBH4+7Na2SiO3xe2x80x83xe2x80x83(2) 
The primary energy cost of these processes stems from the requirement for a large excess of sodium metal (e.g., 4 moles of sodium per mole of sodium borohydride produced). Sodium metal is commercially produced by electrolysis of sodium chloride with an energy input equivalent to about 17,566 BTU (18,528 KJ) per pound of sodium borohydride produced. In contrast, the hydrogen energy stored in borohydride is about 10,752 BTU (11,341 KJ) of hydrogen per pound of sodium borohydride. The Schlesinger process and the Bayer process, therefore, do not provide a favorable energy balance, because the energy cost for producing sodium significantly outweighs the energy provided from sodium borohydride as a fuel.
Furthermore, in view of the large quantities of borohydride needed for use, e.g., in the transportation industry, these processes would also produce large quantities of waste products such as NaOCH3 or Na2SiO3. Since these byproducts are not reclaimed or reused, further energy and expense is required to separate and dispose of these by-products.
Improvements found in the prior art are basically simple modifications of the Schlesinger and Bayer processes represented by equations (1) and (2). Accordingly, such improvements also suffer from the disadvantages stated above, and do not provide any improved energy efficiency or an environmentally sensitive option for disposal of the by-products.
In view of the above, there is a need for improved and energy efficient industrial scale manufacturing processes for producing borohydride compounds. In addition, there is a need for industrial scale processes that reduce or avoid the production of large quantities of waste products that require further disposal.
Accordingly, it is an object of the present invention to provide industrial processes of producing borohydrides with improved energy efficiency. It is also an object of the present invention to provide processes of producing borohydride with reduced levels of unwanted waste products.
The present invention provides processes for producing large quantities of borohydride compounds, which overcome the above-described deficiencies. In addition, the efficiency of the processes of the present invention can be greatly enhanced over the typical processes for producing borohydride compounds.
In one embodiment of the present invention, a process is provided for producing borohydride compounds, which includes: (A) reacting carbon dioxide and water with a Y-containing compound (i.e., a metaborate compound) of formula YBO2 to obtain a bicarbonate compound of the formula YHCO3 and boron oxide; (B) converting YHCO3 into Y2O, carbon dioxide, and water; (C) reacting the boron oxide with carbon and a halide compound of formula X2 to obtain BX3 and carbon monoxide; (D) reacting the BX3 with hydrogen to obtain diborane and HX; and (E) reacting the Y2O with diborane to obtain YBO2 and YBH4. In accordance with the invention, Y is a monovalent cationic moiety such as an alkali metal (e.g., H, Li, Na, K, Rb, Cs, and Fr), a pseudo-alkali metal (e.g., T1), an ammonium ion (NH4+), or a quaternary amine of formula NR4+; R is independently hydrogen, or straight or branched C1 to C4 alkyl group; and X is a halide (F, Cl, Br, I, or At).
In another embodiment of the present invention, a process is provided for producing borohydride compounds, which includes substituting steps (B1) and (E1) for steps (B) and (E): where (B1) entails reacting the YHCO3 to produce Y2CO3, carbon dioxide, and water; and (E1) entails reacting the Y2CO3 with diborane to produce YBH4, YBO2, and carbon dioxide. Y and X are the same as defined above.